Las Vegas casino mogul Sheldon Adelson has donated $1.5 million to the political action committee trying to help Republican George Allen reclaim the U.S. Senate seat he lost six years ago.

Sheldon Adelson and his wife Miriam. (Roslan Rahman/AFP/Getty Images)

The political donation is the second largest since the Virginia Public Access project started tracking money in state politics in 1997. The largest was the $2 million check that now-Sen. Mark Warner (D) wrote to his gubernatorial campaign in 2001, VPAP said.

Independence Virginia has already spent about $2.3 million to help Allen defeat fellow former Virginia governor Timothy M. Kaine (D) in the race to succeed retiring Sen. Jim Webb (D), according to the Center for Responsive Politics.

That figure does not include Adelson’s donation. Until now, the biggest donor to the PAC was Texas home-builder Bob Perry, who has given $1 million. Perry helped bankroll Swift Boat Veterans for Truth against Democrat John Kerry in the 2004 presidential campaign.

This is hardly the first time that Adelson has opened his checkbook this election season. Initially a Newt Gingrich supporter, Adelson has pledged to give Republican Mitt Romney $100 million to unseat President Obama.

Adelson’s donation to Independence Virginia promises to pump up spending in a race — one of a handful that could tip the balance of power in the chamber — that has already been flush with outside cash. The Kaine-Allen contest is behind only the presidential race when it comes to spending by outside groups, which have lavished nearly $32.7 million on the campaign, according to the Center for Responsive politics.

Allen-friendly PACs have outspent their pro-Kaine counterparts by about $2 million, according to the center’s figures.

VPAP, which downloads FEC data daily, shows even higher outside spending on the race and a more pronounced edge for Allen. Pro-Allen/anti-Kaine PACs have spent $21.5 million, while pro-Kaine/anti-Allen ones have spent $14.4 million, VPAP says.

Allen spokeswoman Emily Davis declined to comment on Adelson’s donation. By law, the campaign cannot coordinate with the group.

Kaine spokeswoman Brandi Hoffine said via e-mail: “Last time Virginians sent George Allen to the senate, he put tax breaks for the wealthy and corporations who ship jobs overseas ahead of Virginia's best interests and paved the way for our current fiscal and economic mess.

“Given that record, it's not surprising that an out-of-state casino magnate with an interest in maintaining those loopholes and continuing to enjoy absurdly low tax rates like the 1% income tax Mr. Adelson paid in 2009 are backing George Allen's campaign.”